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Datum Lettering Sequence A,B,C or E,K,Z ?

Datum Lettering Sequence A,B,C or E,K,Z ?

Datum Lettering Sequence A,B,C or E,K,Z ?

It is written anywhere in 82 or 94 that it is manditory to start your datum lettering sequence as A,B,C without skipping letters or is it allowed to use any arrangement of letters on a drawing?

RE: Datum Lettering Sequence A,B,C or E,K,Z ?

ASME Y14.5M-1994, paragraph 3.2 "Datum Feature Symbol" states the following with regard t datum feature letters:

“… Letters of the alphabet (except I, O, and Q) are used as datum identifying letters. Each datum feature of a part requiring identification shall be assigned a different letter. When datum features requiring identification on a drawing are so numerous as to exhaust the single alpha series, the double alpha series (AA through AZ, BA through BZ, etc.) shall be used and enclosed
in a rectangular frame. ….”

The paragraph makes no mention that you must use the letters is alphabetical order.  Therefore, you are free to mix and match the letters as you please (except for the letters I, O and Q).  

However, this becomes risky when you have a drawing that contains many datum reference callouts.  Without tracking which letters have already been used, you run the risk of someone duplicating a letter when they add a new datum to the part.  If applied in order alphabetically, you can better track the letters that have already been used and avoid this duplication.

Hope this helps


RE: Datum Lettering Sequence A,B,C or E,K,Z ?

GDTGUY is on the money. One thing I like to do is make sure that the letters are not reused in sections, auxiliary views or datum's. I tend to require only a few of each, so I start the sections (or views) with "A" and the datum’s from "Z".

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